1. Field of the Invention
The invention relates to a control device.
2. Description of the Related Art
In regard to signal processing techniques for a compression region of a power amplifier (hereinafter called PA) in a high-speed radio communications, a digital predistortion approach can compensate the state of PA linearity and reduce hardware cost, especially, in the applications of digital signal processors. The digital predistortion scheme is for compensating transmission signals in advance for the distortion caused by the power amplifier.
FIG. 1 is a diagram of a conventional predistorter 11 and a conventional power amplifier 12. A graph (A) in the bottom-left portion of FIG. 1 shows a transfer characteristic curve of input power versus output power of predistortion compensation; a graph (B) in the bottom-middle portion of FIG. 1 is a transfer characteristic curve of input power versus output power of a power amplifier; a graph (C) in the bottom-right portion of FIG. 1 shows a transfer characteristic curve of input power versus output power of the power amplifier with predistortion. The predistorter unit 11 supplies a predistortion signal P to the power amplifier 12 such that a linear amplification (as shown in graph (C)) of the input signal of the power amplifier is achieved by combining the two transfer characteristics (as shown in graphs (A) and (B)).
The power amplifier operates in a linear region, a compression region, or a saturation region according to the power magnitude of the input signal. In a situation, the power magnitude of the input signal is too large such that the power amplifier operates in the saturation region and the predistortion cannot compensate the signal distortion. Thus, it is very important to appropriately confine the power magnitude of the input signal to the compression region and the linear region of the power amplifier.